From Miller and others (1998) [1]: "Aniakchak Crater is an ice-free, circular caldera about 10 km in diameter and a maximum of 1 km deep which was first described by Smith (1925) [2]. The pre-caldera cone was built upon a basement of Tertiary sedimentary and volcanic rocks and Jurassic-Cretaceous sedimentary rocks, which are exposed high on the east and south walls of the caldera [3]. The elevation of the caldera rim varies from 1,341 m to 610 m. Surprise Lake, a 3.2-km-long lake in the northeast part of the caldera at an elevation of about 335 m is the source of the Aniakchak River, which flows through a breach in the eastern wall of the caldera. Numerous domes, flows, and cones occupy the interior of the caldera [4]; the largest cone is Vent Mountain, 2.5 km in diameter and rising 430 m above the floor of the caldera. The pre-caldera cone was built on the west side of a basement high. The cone was deeply dissected by numerous glaciers that cut U-shaped valleys into the slopes before the caldera-forming eruption.

From Bacon and others (2014) [4]: “The oldest recognized postglacial explosive eruption, andesitic Aniakchak I, left nonwelded ignimbrite in valleys below the edifice and fines-poor welded ignimbrite or agglutinated fall deposits high on its flanks between ca. 9,500 and 7,000 years ago. A small caldera may have collapsed at the source of the ignimbrite, likely just west of the present Vent Mountain…Subsequent to Aniakchak I, Plinian eruptions ca. 7,000 14C yr B.P. from a vent northeast of the edifice summit produced the Black Nose Pumice, consisting of a lower unit of rhyodacite lava, pumice fall, and intraplinian welded ignimbrite and an upper unit of dacite pumice fall and northeast flank lava flow. At least 20 additional Holocene eruptions are thought to have occurred before the Aniakchak II caldera-forming event.”

From Miller and others (1999) [1]: "Ash flows from the caldera-forming eruption - 3430 +/- 10 yrs B.P. [5] - reached both the Bering Sea and the Pacific Ocean [6]. They are typically non-welded and fill glacial valleys to a depth of at least 75 m adjacent to the caldera rim. The ash flows were highly mobile, over-running 260-meter-high passes in the Aleutian Range and traveling as far as 50 km from the caldera rim [6]."

From Bacon and others (2014) [4]: “Postcaldera vents are mainly on the caldera ring-fracture system. The earliest extruded small dacite domes into a deep caldera lake and a lava flow on the northwest flank. Three basaltic andesite-andesite tuff cones were constructed on the eastern caldera floor after catastrophic draining of the lake by ~200 m. Dacite-andesite magmas issued from Vent Mountain and Half Cone starting as early as ~1,000 years ago. Plinian eruption at Half Cone ~400 14C yr B.P. yielded widespread dacite Pink and overlying andesite Brown Pumice fall deposits. Strombolian eruption of basaltic andesite built Blocky Cone after Half Cone and most Vent Mountain activity. The most recent eruption, in 1931, yielded dacite-rhyodacite tephra followed by relatively voluminous andesite tephra and ended with minor basaltic andesite.”